try turning it on and as soon as it turns on start tapping the F11 key and it might take you into the recovery manager from where you can restore it back to factory settings
(I say might because you've not said what thinkpad and also because it's possible that the recovery partition might have been removed at some point in the past)
(and don't worry about the CD drive, windows has drivers built in for CD drives)
if chucks advice does not work you can get a copy Windows xp from ebay for about £45 (if you have not got one!). Installing a new copy gives you the option of formatting the drive.